WASHINGTON (CNN) - California congresswoman Ellen Tauscher is vacating her Bay Area seat to serve under Hillary Clinton at the State Department.

Tauscher, a moderate Democrat and a member of the House Armed Services Committee, said in a message to her consituents that Clinton had asked her to serve as undersecretary of state for arms control and international security.

She said she accepted the post after 13 years in Congress "after much soul-searching."

"Keeping nuclear weapons out of the hands of terrorists, making sure other countries do not obtain them and, one day, I hope, ridding the world of these terrible weapons, has become my passion and, I hope, my life's work," she said in a statement.

If she is confirmed for the post, Tauscher's departure will almost certainly be followed by a crowded Democratic primary in the left-leaning California district.